1. Field of the Invention
The present invention is for a device to facilitate sublistatic printing and includes an elastic member for holding a sublistatic transfer sheet against an article surface to be imprinted which is applied while in a stretched condition. The invention also includes means for stretching the elastic member and applying it to the article surface so that it holds the sublistatic transfer sheet in intimate contact against the article surface to be imprinted.
2. Background Information
Sublistatic printing is the technique of first placing a dye in the proper form and color on a vehicle, for example, a piece of paper conveniently referred to as a transfer sheet. The dye consists of special particles in an appropriate carrier such as water to form a viscous liquid which can be transferred to the transfer sheet by conventional printing equipment such as roller or offset printing machines. The dye can be in the form of a dry powder to be transferred to the transfer sheet using electrostatic principals such as those used in paper copiers. In all cases, however, the basic dye is manufactured from a material that sublimes; i.e., the material can convert from a solid particle to a gas without going through a liquid phase. Such a conversion is usually initiated by the application of heat. Thus positioning a transfer sheet against an article surface to be imprinted and applying heat will result in sublistatically printing that article surface. Detailed disclosures of the sublistatic printing process are set forth in U.S. Pat. Nos. 4,246,331 and 4,342,281, the contents of which are incorporated herein by reference.
Many materials have surface properties that cause them to expand upon the application of heat and then contract with cooling. During sublistatic printing, the heat forms microscopic tunnels or voids in the material surface, and the dye vapor moves into these voids by molecular dispersion. When the heat is removed and the surface cools, the microscopic tunnels or voids contract and close so that the trapped dye can be observed through the semi-transparent surface material which makes the dye coloring as permanent as the object surface.
There are many materials which are not deformable by heat or pressure and therefore not in their natural state susceptible to sublistatic printing. In those cases, the object surface must be coated with a material that does possess those desirable characteristics.
Many articles manufactured of materials not in their natural state susceptible to sublistatic printing are those upon which sublistatic printing is most desirable. Coffee cups, glasses, dishes, bowls and other articles of similar materials having arcuate surfaces are representative. Even though the forms in which they are cast are accurately machined, the soft clay used in their formation tends to slump and deform slightly before it can harden. Complete positive (intimate) contact under pressure between the transfer sheet and the surface to be imprinted is necessary for satisfactory sublistatic printing, and the surface irregularities of clay made products even when coated with a print inducing material make such contact difficult.
Applying the necessary heat to arcuate surfaces to be sublistatically imprinted has also been a difficult undertaking in the past. Heaters for this procedure have been usually of a sandwich construction generally consisting of a layer of fiberglass cloth, a reinforced silicone rubber sheet with a resistence heating element covered by another reinforced silicone sheet. This assembly is somewhat flexible but cannot be easily deformed locally to follow small rises and depressions in the surface to be imprinted and thereby achieve the intimate contact necessary.
Prior methods to apply circumferential or radial force to make the heater conform to surface irregularities include positioning wraps or webbing around the heater, transfer sheet and article and then tensioning the wraps or webbing to more evenly distribute the strap pressure to the irregularly shaped article surface.
Problems associated with these methods for conducting sublistatic printing on articles having arcuate surfaces generally are caused by trying to achieve an even pressure to all points of a curved surface by use of a strap or straps around the heater and transfer sheet to hold it against the article surface to be imprinted through pulling only from the ends of the strap or straps. Here the greatest radial pressure occurs near the pulling action - the strap ends. The remaining area covered by the strap receives less force from the pull. Therefore, radial pressure differentials exist at different points along the circumference of the article surface. In this situation, the sublistatic print will be darker in areas of high pressure and lighter in areas of low pressure.
The positioning of the transfer sheet on arcuate surfaces has also been a problem in the past in that the transfer sheet usually had to be taped to the surface prior to the application of a pressure applying device. This obviously takes a significant amount of time and effort and presents difficulties in achieving precise alignment during each installation.